Micromirror unit with torsion connector having nonconstant width

ABSTRACT

A micromirror unit is provided which includes a frame, a mirror forming base upon which a mirror surface is formed, and a torsion connector which includes a first end connected to the mirror forming base and a second end connected to the frame. The torsion connector defines a rotation axis about which the mirror forming base is rotated relative to the frame. The torsion connector has a width measured in a direction which is parallel to the mirror surface and perpendicular to the rotation axis. The width of the torsion connector is relatively great at the first end. The width becomes gradually smaller from the first end toward the second end.

This application is a continuation application of application Ser. No.10/766,040, filed Jan. 29, 2004, now U.S. Pat. No. 7,130,099, which wasa continuation application of co-pending application Ser. No.09/984,814, filed on Oct. 31, 2001, now U.S. Pat. No. 6,795,225, theentire contents and disclosure of which are entirely incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a micromirror unit used in opticalapparatus for the purposes of changing the direction of light. Inparticular, it relates to a micromirror unit of the type which isadvantageously incorporated in an optical disk apparatus (for writing toor reading data from an optical disk), an optical switching apparatus(for selectively connecting one optical fiber to another to provide alight passage), etc.

2. Description of the Related Art

A micromirror unit is provided with a reflective mirror member which ispivotable for changing the direction of reflected light. A populartechnique for actuating the mirror member is to utilize electrostaticforce. Micromirror units of this type (referred to as “static drivingtype” hereinafter) may have several structures. Such micromirror unitsare generally classified into two groups, depending on fabricationmethods. One of the methods employs a “surface micro-machining”technique, whereas the other employs a “bulk micro-machining” technique.

In accordance with the surface micro-machining, patterned materiallayers in lamination may be formed on a base substrate, therebyproviding required components such as a support, a mirror member andelectrodes. In this layer forming process, a dummy layer (or sacrificiallayer), which will be removed later, may also be formed on thesubstrate. A conventional micromirror unit of the static driving type bythe surface micro-machining is disclosed in JP-A-7 (1995)-287177 forexample.

In accordance with the bulk micro-machining, on the other hand, a basesubstrate itself is subjected to etching, thereby providing requiredcomponents such as a frame and a mirror forming base. Then, a mirrormember and electrodes may be formed on the etched substrate by athin-film forming technique. Micromirror units of the static drivingtype by the bulk micro-machining are disclosed in JP-A-9 (1997)-146032,JP-A-9-146034, JP-A-10 (1998)-62709 and JP-A-2000-13443.

One of the technically significant factors desired in a micromirror unitis a high flatness of the reflective mirror member. According to theabove-mentioned surface micro-machining technique, however, thethickness of the resulting mirror member is rendered very small, so thatthe mirror member is liable to warp. To avoid this and ensure a highflatness, the mirror member should be made so small that its respectiveedges are less than 100 .mu.m in length. In accordance with the bulkmicro-machining, on the other hand, a rather thick substrate isprocessed, thereby providing a sufficiently rigid mirror forming base tosupport the mirror member. Thus, a relatively large mirror member havinga high flatness can be obtained. Due to this advantage, the bulkmicro-machining technique is widely used to fabricate a micromirror unithaving a large mirror member whose edges are more than 100 .mu.m inlength.

FIG. 10 of the accompanying drawings shows an example of conventionalmicromirror unit fabricated by the bulk micro-machining technique. Theillustrated micromirror unit 400 is of the static driving type, andincludes a lamination of a mirror substrate 410 and a base substrate420. As shown in FIG. 11, the mirror substrate 410 includes a mirrorforming base 411 and a frame 413. The mirror forming base 411 has anobverse surface upon which a mirror member 411 a is formed. The mirrorforming base 411 is supported by the frame 413 via a pair of torsionbars 412. The mirror forming base 411 has an reverse surface upon whicha pair of electrodes 414 a and 414 b is formed. As shown in FIG. 10, thebase substrate 420 is provided with a pair of electrodes 421 a and 421 bwhich faces the above-mentioned pair of electrodes 414 a and 414 b ofthe mirror forming base 411.

With the above arrangement, the electrodes 414 a, 414 b of the mirrorforming base 411 may be positively charged, whereas the electrode 421 aof the base substrate 420 may be negatively charged. As a result, anelectrostatic force is generated between these electrodes, therebyturning the mirror forming base 411 in the N3-direction shown in FIG. 10as the torsion bars 412 are being twisted. The rotation angle of themirror forming base 411 is determined by the balance between theinter-electrode electrostatic force and the restoring force of thetwisted torsion bars 412. To rotate the mirror forming base 411 in theopposite direction, the other electrode 421 b of the substrate 420 maybe negatively charged. As readily understood, when the mirror formingbase 411 is turned clockwise or counterclockwise, as required, the lightreflected on the mirror member 411 a is directed in the desireddirection.

As noted above, the mirror forming base 411 is rotated through an anglewhich is defined by the balance between the inter-electrodeelectrostatic force and the restoring force of the twisted torsion bars412. Thus, it is possible to adjust the rotation angle of the base 411by controlling the static electricity to be generated in correlationwith the restoring force of the torsion bars 412.

Generally, a micromirror unit is a structure whose minimum dimension isabout several hundred micrometers. This is rather large size, andtherefore the restoring force of the torsion bars tends to exceed theinter-electrode electrostatic force in strength. Thus, conventionally,the area of each electrode is rendered large (for generating a greatelectrostatic force), whereas each torsion bar is made uniformly thinalong its length (for weakening the restoring force). In the prior artmicromirror unit 410 (FIG. 11), each torsion bar 412 has a constantsmall width L along the entire length.

In the above manner, however, the mirror forming base 411 is supportedby the thin torsion bars 412. Accordingly, it is difficult to hold themirror forming base 411 stable (i.e., nonrotatable) about the normal N3(the line at right angles to the surface). If unstable about the normalN3, the mirror forming base 411 is liable to unduly swivel about thenormal N3 when the base 411 is supposed to rotate only about the axisdefined by the torsion bars 412. When such an unwanted swivel occurs, itis difficult or even impossible to precisely control the operation ofthe micromirror unit.

SUMMARY OF THE INVENTION

The present invention has been proposed under the circumstancesdescribed above. It is, therefore, an object of the present invention toprovide a micromirror unit which does not suffer from the above-notedproblems. Specifically, an object of the present invention is to providea micromirror unit which is provided with torsion bars of reducedrestoring force and still can exert excellent stability againstundesired swiveling.

According to a first aspect of the present invention, there is provideda micromirror unit which includes: a first frame; a mirror forming baseprovided with a mirror surface; and a first torsion connector whichincludes a first end connected to the mirror forming base and a secondend connected to the first frame. The torsion connector defines a firstaxis about which the mirror forming base is rotated relative to thefirst frame. The torsion connector has a width measured in a directionwhich is parallel to the mirror surface and perpendicular to the firstaxis. The width of the first torsion connector is relatively great atthe first end and becomes gradually smaller from the first end towardthe second end.

In a preferred embodiment, a micromirror unit further includes a secondframe and a second torsion connector. The second torsion connectorconnects the second frame to the first frame and defines a second axisabout which the first frame and the mirror forming base are rotatedrelative to the second frame.

In another preferred embodiment, the second torsion connector has awidth measured in a direction which is parallel to the mirror surfaceand perpendicular to the second axis, wherein the width of the secondtorsion connector is relatively great at a connecting portion to thefirst frame, and becomes gradually smaller from the first frame towardthe second frame.

Preferably, the first torsion connector may include a plurality oftorsion bars.

Preferably, a micromirror unit may further include a first potentialconducting path and a second potential conducting path, wherein each ofthe torsion bars is connected to one of the first and the secondpotential conducting paths.

Preferably, the width of the first torsion connector becomesmonotonically smaller from the first end to the second end.

In a preferred embodiment, the first torsion connector includes anintermediate point between the first end and the second end. The widthof the first torsion connector becomes monotonically smaller from thefirst end to the intermediate point and becomes monotonically greaterfrom the intermediate point to the second end.

Preferably, the first torsion connector has a rectangular cross sectionor a circular cross section or an elliptical cross section.

Preferably, the first torsion connector has a hollow structure.

Preferably, the first torsion connector includes a bifurcating portion.

Preferably, the first torsion connector may include, in at least one ofthe first end and the second end, a curved portion for prevention ofstress concentration.

In a preferred embodiment, the mirror forming base is provided with afirst comb-teeth electrode, while the first frame is provided with asecond comb-teeth electrode cooperating with the first comb-teethelectrode for moving the mirror forming base.

Preferably, a micromirror unit may further include a support base facingthe mirror forming base. The support base is provided with a firstelectrode facing the mirror forming base, while the mirror forming baseis provided with a second electrode facing the first electrode.

Preferably, the mirror forming base may be provided with a firstelectromagnetic coil, and the support base may be provided with a secondelectromagnetic coil or a permanent magnet facing the firstelectromagnetic coil.

Preferably, the mirror forming base may be provided with a permanentmagnet, and the support base may be provided with an electromagneticcoil facing the permanent magnet.

Preferably, at least a part of the first frame may have a multi-layerstructure including a plurality of conductive layers and an insulatinglayer disposed between the conductive layers.

Preferably, the first frame may be provided with a third comb-teethelectrode, and the second frame may be provided with a fourth comb-teethelectrode cooperating with the third comb-teeth electrode for moving thefirst frame and the mirror forming base.

According to a second aspect of the present invention, there is provideda micromirror unit which includes: an inner frame; an outer frame; amirror forming base provided with a mirror surface; an inner torsionconnector connecting the inner frame to the mirror forming base; and anouter torsion connector which connects the inner frame to the outerframe and defines an axis about which the inner frame and the mirrorforming base are rotated relative to the outer frame. The outer torsionconnector has a width measured in a direction which is parallel to themirror surface and perpendicular to said axis. The width of the outertorsion connector is relatively great at a connecting portion to theinner frame, and becomes gradually smaller from the inner frame and tothe outer frame.

Other features and advantages of the present invention will becomeapparent from the detailed description given below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view showing a micromirror unit according to afirst embodiment of the present invention;

FIG. 2 is a sectional side view showing the assembled state of themicromirror unit of FIG. 1;

FIG. 3A is an upper plan view showing a micromirror unit according to asecond embodiment of the present invention;

FIG. 3B is a lower plan view showing the micromirror unit of FIG. 3A;

FIG. 4A is a sectional view taken along lines A-A in FIG. 3A or 3B;

FIG. 4B is a sectional view taken along lines B-B in FIG. 3A or 3B;

FIG. 4C is a sectional view taken along lines C-C in FIG. 3A or 3B;

FIGS. 5A-5H and 6A-6E are sectional views, showing a fabrication methodof the micromirror unit of FIG. 3, which are taken along lines E-E inFIG. 3A or 3B;

FIGS. 7A and 7B are plan views showing the configuration ofpattern-forming masks used for the fabrication procedure shown in FIG.5;

FIGS. 8A and 8B are plan views showing the configuration ofpattern-forming masks used for the fabrication procedure shown in FIG.6;

FIGS. 9A-9I show, in plan and section, examples of torsion connectorsadoptable in a micromirror unit embodying the present invention;

FIG. 10 is a sectional view showing a conventional micromirror unit; and

FIG. 11 is a perspective view showing the conventional micromirror unitof FIG. 10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiments of the present invention will be describedbelow with reference to the accompanying drawings.

FIGS. 1 and 2 show a micromirror unit 100 according to a firstembodiment of the present invention. The illustrated unit 100 is a“static driving type” device, and includes two superposed substrates,i.e., a mirror substrate 110 and a base substrate 120.

As shown in FIG. 1, the mirror substrate 110 includes a mirror formingbase 111, a frame 113 around the base 111, and a pair of torsionconnectors 112 connecting the base 111 to the frame 113. The mirrorsubstrate 110 may be made of a conductive silicon material doped withn-type impurity (such as phosphorous or arsenic) or p-type impurity(e.g. boron). The mirror substrate 110 may be fabricated by a bulkmicro-machining technique. Specifically, first a plate of conductivesilicon substrate is prepared. Then, for forming several openings 110 a(see the figure), portions of the silicon plate that correspond to themirror forming base 111, the frame 113 and the torsion connectors 112are covered by an etching mask. Finally, the masked silicon plate issubjected to Si etching (by Deep RIE) or wet Si etching (by KOH). Thethus formed openings 110 a define the configurations of the mirrorforming base 111, the frame 113 and the torsion connectors 112. In theillustrated example, each opening 110 a (between the mirror forming base111 and the frame 113) has a width ranging from 10 μm to 200 μm forexample, while the thickness of the mirror forming base 111 and frame113 ranges from 10 μm to 200 μm.

As shown in FIG. 2, the obverse surface of the mirror forming base 111is provided with a mirror member 114, while its reverse surface isprovided with a pair of electrodes 115 a, 115 b. The mirror member 114and the electrodes 115 a, 115 b may be made by vapor deposition ofmetal. The electrodes 115 a, 115 b may be omitted when the conductivityof the mirror substrate 110 is sufficiently high (this can be achievedby doping the mirror substrate 110 with impurities).

As shown in FIG. 1, each of the torsion connectors 112 is integrallyconnected, at one end, to the center of a longitudinal side surface ofthe mirror forming base 111, and at the other end, to the center of aninner longitudinal side surface of the frame 113. This structure makesthe micromirror unit 100 of the preferred embodiment a single-axis typedevice in which the paired torsion connectors 112 define a rotation axisX1. In the illustrated embodiment, each torsion connector 112 includestwo torsion bars 112 a. These two torsion bars 112 a define the ‘width’of the torsion connector 112, where the ‘width’ refers to the dimensionmeasured in the Y-direction shown in FIG. 1. By this definition, thetorsion connector 112 has a relatively great width of 30˜300 μm at itsinner end (where the connector 112 is fixed to the base 111), andbecomes narrower gradually from the mirror forming base 111 toward theframe 113. At the narrowest portion (where the connector 112 is fixed tothe frame 113), the torsion connector 112 has a width of 1˜30 μm.

As shown in FIG. 2, the frame 113 is attached to the upwardly protrudingrim 121 of the base substrate 120. The base substrate 120 is providedwith a pair of electrodes 122 a, 122 b which face the electrodes 115 a,115 b of the mirror forming base 111. This makes the micromirror unit100 a “parallel electrode type” device.

With the above arrangement, the electrodes 115 a, 115 b of the mirrorforming base 111 may be positively charged, while the electrode 122 a ofthe base substrate 120 may be negatively charged. As a result, anelectrostatic force is generated between these electrodes, therebyturning the mirror forming base 111 in the N1-direction against therestoring force of the paired torsion connectors 112. To turn the mirrorforming base 111 in the opposite direction, the electrode 122 b may benegatively charged. As the mirror forming base 111 is turned clockwiseor counter-clockwise, the light reflected on the mirror member 114 canbe directed in a desired direction.

In the preferred embodiment, the mirror forming base 111 is smoothlyturned about the axis X1 (FIG. 1) with application of an advantageouslysmall driving voltage. This is because each torsion connector 112 has amechanically weaker portion (narrower portion) against the twistingforce acting on the mirror forming base 111. At the same time, eachtorsion connector 112 is connected to the mirror forming base 111 at itsmaximum width portion. Thus, the swiveling of the base 111 about thenormal N1 is properly prevented.

The positive potential is applied to the electrodes 115 a, 115 b of themirror forming base 111 via the frame 113, torsion connectors 112 andmirror forming base 111, all of which are integrally made of conductivematerial. In this way, it is possible to apply the necessary potentialto the electrodes 115 a, 115 b on the mirror forming base 111 withoutproviding an additional wiring pattern on the torsion connectors 112 andother elements. On the other hand, the negative potential is applied tothe electrodes 122 a, 122 b of the base substrate 120 via a wiringpattern (not shown) formed on the base substrate 120. The base substrateitself is made of insulating material.

Instead of using the above-described parallel electrode strategy, usemay be made of comb-teeth electrodes for actuating the mirror formingbase 111 of the micromirror unit 100. Also, instead of usingelectrostatic force, use may be made of an attracting or repelling forcethat results from permanent magnets or electromagnets. Specifically,each of the electrodes 115 a, 115 b on the mirror forming base 111 maybe replaced by an electromagnetic coil, while each of the electrodes 122a, 122 b on the base substrate may be replaced by an electromagneticcoil or permanent magnet. Alternatively, each of the electrodes 115 a,115 b on the mirror forming base 111 may be replaced by a permanentmagnet, while each of the electrodes 122 a, 122 b on the base substratemay be replaced by an electromagnetic coil. In these arrangements, it ispossible to control the rotation of the mirror forming base 111 byvarying the potential applied to the electromagnetic coil.

Reference is now made to FIGS. 3A-3B and 4A-4C illustrating amicromirror unit 200 according to a second embodiment of the presentinvention. The upper view of the unit 200 is shown in FIG. 3A, while thebottom view is shown in FIG. 3B. FIGS. 4A, 4B and 4C are sectional viewstaken along lines A-A, B-B and C-C in FIG. 3, respectively.

As shown in FIGS. 3A and 3B, the micromirror unit 200 of the secondembodiment includes a mirror forming base 210, an inner frame 220surrounding the base 210, an outer frame 230 surrounding the inner frame220, a pair of first torsion connectors 240 connecting the mirrorforming base 210 to the inner frame 220, and a pair of second torsionconnectors 250 connecting the inner frame 220 to the outer frame 230.The first torsion connectors 240 have a first rotation axis X2 aboutwhich the mirror forming base 210 is rotated with respect to the innerframe 220. The second torsion connectors 250 have a second rotation axisX3 about which the inner frame 220 is rotated with respect to the outerframe 230. In the illustrated embodiment, the first axis X2 isperpendicular to the second axis X3. All the components of themicromirror unit 200 are made of a conductive material except a mirrormember 211 and an insulating layer 260, as will be described below. Theconductive material may be a semiconductor (e.g. Si) doped with n-typeimpurity (e.g. phosphorous or arsenic) or p-type impurity (e.g. boron).Alternatively, a metal (tungsten) may be used for the conductivematerial.

As shown in FIG. 3A, the mirror forming base 210 is a rectangular plate,having an upper surface upon which a thin reflective layer (mirrormember) 211 is provided. The mirror forming base 210 has two relativelylong side surfaces and two relatively short side surfaces. The mirrorforming base 210 is provided with two sets of first comb-teethelectrodes 210 a, 210 b extending outward from the shorter side surfacesof the mirror forming base 210.

The inner frame 220, as shown in FIGS. 3B and 4, includes a frame body221 and a pair of electrode bases 222. Each of the electrode base 222 isattached to the frame body 221 with an insulating layer 260 interveningtherebetween for electrical insulation. The electrode bases 222 areprovided with second comb-teeth electrodes 222 a or 222 b extendinginward. The frame body 221 is provided with third comb-teeth electrodes221 a, 221 b extending outward. As best shown in FIG. 4A, the secondcomb-teeth electrodes 222 a, 222 b are disposed under the firstcomb-teeth electrodes 210 a or 210 b. In addition, as shown in FIG. 4C,the first comb-teeth electrodes 210 a (or 210 b) are horizontally offsetfrom the second comb-teeth electrodes 222 a (or 222 b) so that they willnot interfere when the mirror forming base 210 is caused to pivot aboutthe first torsion connectors 240.

As shown in FIG. 3A or 3B, each first torsion connector 240 includes twobifurcating torsion bars 241. In this instance again, the width of thetorsion connector 240, that is defined by these two torsion bars 241,becomes gradually smaller from the mirror forming base 210 toward theinner frame 220. Specifically, the greatest width of the connector 240(the portion connected to the mirror forming base 210) may be in a rangeof 30˜300 μm, while the smallest width of the connector 240 (the portionconnected to the inner frame 220) may be in a range of 1˜30 μm. As shownin FIG. 4B, the torsion bars 241 are smaller in thickness than themirror forming base 210 and the inner frame 220.

As best shown in FIG. 4A, the outer frame 230 includes a first or upperframe member 231 and a second or lower frame member 232. The first andthe second frame members 231, 232 are electrically insulated from eachother by an insulating layer 260 disposed between the two frame members.As shown in FIG. 3B, the second frame member 232 is provided with afirst auxiliary strip (or first island) 233, a second auxiliary strip(or second island) 234, a third auxiliary strip (or third island) 235and a fourth auxiliary strip (or fourth island) 236. The first to thefourth islands 233-236 are spaced from each other for electricalinsulation. As shown in FIGS. 3B and 4B, the first island 233 and thethird island 235 are formed integral with fourth comb-teeth electrodes232 a, 232 b extending inward. The fourth comb-teeth electrodes 232 a,232 b are located below and laterally offset from the third comb-teethelectrodes 221 a, 222 b. With such an offset, the third comb-teethelectrodes 221 a, 221 b do not interfere with the fourth comb-teethelectrodes 222 a, 222 b even when the inner frame 220 is turned.

As shown in FIGS. 3A and 3B, the second torsion connectors 250 eachinclude one central torsion bar 251 and two nonparallel, outer torsionbars 252. The width of each connector 250 is defined by the outertorsion bars 252. The greatest value of the width may be 30˜300 μm,while the smallest value of the width may be 1˜30 μm. As shown in FIG.4A, the torsion bars 251 and 252 are smaller in thickness than the innerand the outer frames 220, 230. The central torsion bar 251 bridgesbetween the body 221 of the inner frame 220 and the first frame member231 of the outer frame 230. The other torsion bars 252 bridge betweenthe electrode base 222 of the inner frame 220 and the second framemember 232 of the outer frame 230.

With the above arrangements, when a potential is applied to the upperframe member 231, the effect is conducted to the first comb-teethelectrodes 210 a-210 b and the third comb-teeth electrodes 221 a-221 bvia the torsion bars 251, the inner frame body 221, the first torsionconnectors 240 or four torsion bars 241 and the mirror forming base 210.As a result, the first comb-teeth electrodes 210 a, 210 b and the thirdcomb-teeth electrodes 221 a, 221 b are held at the same potential. Inthis state, when the second comb-teeth electrodes 222 a, 222 b arecharged to a desired potential, an electrostatic force is generatedbetween the first comb-teeth electrodes 210 a or 210 b and the secondcomb-teeth electrodes 222 a or 222 b. As a result, the mirror formingbase 210 is turned about the rotation axis X2. Likewise, when the fourthcomb-teeth electrodes 232 a, 232 b are charged to a desired potential,an electrostatic force is generated between the third comb-teethelectrodes 221 a or 221 b and the fourth comb-teeth electrodes 232 a or232 b. As a result, the inner frame 220 together with the mirror formingbase 210 is turned about the rotation axis X3.

As seen from FIG. 4A, the application of potential to the secondcomb-teeth electrodes 222 a is performed through the fourth island 236,the torsion bar 252 connected to the island 236, the torsion bar 252connected to the island, and the relevant one of the electrode bases222. Likewise, the application of potential to the second comb-teethelectrodes 222 b is performed through the second island 234, the torsionbar 252 connected to the island, and the electrode base 222. As seenfrom FIG. 4B, the application of potential to the fourth comb-teethelectrodes 232 a is performed through the first island 233. Likewise,the application of potential to the fourth comb-teeth electrodes 232 bis performed through the third island 235. Since the four islands233˜236 are electrically insulated from each other, the requiredpotential can be applied selectively to the second comb-teeth electrodes222 a, 222 b or the fourth comb-teeth electrodes 232 a, 232 b.Accordingly, the mirror forming base 210 and hence the mirror member 211can be directed in a desired direction.

In the above-described second embodiment again, the mirror forming base210 can be properly turned about a predetermined axis due to arelatively narrow portion of the first or second torsion connector 240or 250. At the same time, the undesired swiveling of the mirror formingbase 210 about the normal (not shown) is prevented due to the flaringconfiguration of the first and the second torsion connectors 240, 250.

Referring now to FIGS. 5A-5H and 6A-6E, a fabrication method of themicromirror unit 200 of FIG. 3 will be described below. FIGS. 5A-5H and6A-6E are sectional views taken along lines E-E in FIG. 3.

First, as shown in FIG. 5A, two conductive plates 200′ are prepared.These plates may be a silicon wafer doped with n-type impurity such asarsenic or p-type impurity such as boron. Preferably, the doped wafermay have a resistivity of 0.01-0.1 Ω·cm. Each of the conductive plates200′ has its upper surface covered by a silicon dioxide layer 260 of 500nm thickness. This layer may be formed by thermal oxidation.

Then, as shown in FIG. 5B, the two plates 200′ are fixed to each otherwith their silicon dioxide layers 260 held in contact. The fixation maybe achieved by annealing under nitrogen atmosphere with an annealingtemperature of about 1100° C. Then, the attached plates 200′ aresubjected to grinding so that each of them has a thickness of 100 μm. Asa result, an SOI (Silicon on Insulator) assembly is obtained, whichconsists of the upper Si layer 201 a (100 μm in thickness), the SiO2insulator 260′ (1 μm in thickness) and the lower Si layer 201 b (100 μmin thickness).

Then, as shown in FIG. 5C, the exposed surface of the upper Si layer 201a is covered by a silicon dioxide layer 30′ to produce a first etchingmask. The thickness of the layer 30′ may be 100˜1000 nm. At this stage,though not shown in the figure, the exposed surface of the lower Silayer 201 b may also be covered by the same SiO2 layer. As is obvious tothe person skilled in the art, the layer 30′ may be made of othermaterials than silicon dioxide, as long as the alternative material canserve proper masking function when the Si layer 201 a is subjected tothe etching processes by Deep RIE. The layer forming technique may bethermal oxidation, CVD (chemical vapor deposition), etc.

Then, as shown in FIG. 5D, the SiO2 layer 30′ is etched away in theprescribed portions to provide a first mask 30. The patterning for themask 30 is performed with the use of a first mask pattern 40 shown inFIG. 7A. The configuration of the first mask pattern 40 corresponds tothe layout of the principal components of the micromirror unit 200, suchas the mirror forming base 210, the first comb-teeth electrodes 210a-210 b, the inner frame body 221, the third comb-teeth electrodes 221a-221 b, and the upper frame member 231 of the outer frame 230. Thepatterning of the layer 30′ may be performed by wet etching (usinghydrogen fluoride solution) or dry etching (using CHF3 gas, C4F8 gas,etc.).

Then, as shown in FIG. 5E, a second mask 50 is formed on the upper Silayer 201 a. To this end, though not shown in the figure, a photoresistlayer, from which the mask 50 is produced, is formed on the upper Silayer 201′ and then etched into the prescribed pattern. The thickness ofthe photoresist layer may be 0.5˜50 μm. Use may be made of an Si3N4layer in place of the photoresist layer. The layer forming method may bethermal oxidation or CVD for example. The etching of the photoresistlayer is performed with the use of a second mask pattern 60 shown inFIG. 7B. The configuration of the second mask pattern 60 corresponds tothe first torsion connectors 240 (four torsion bars 241), the torsionbars 251 and support beams 270. In the illustrated embodiment, a set offour support beams 270 is provided for connecting the inner frame 220 tothe mirror forming base 210, and another set of four support beams 270is provided for connecting the inner frame 220 to the outer frame 230.The support beams 270 serve to alleviate stress concentration at thefirst and the second torsion connectors in the midst of fabricating themicromirror unit. The etching with the use of the second pattern 60 maybe performed by photo etching, wet etching (using HF solution) or dryetching (using CHF3 gas or C4F8 gas). This etching should be performedunder conditions that do not etch away the first mask pattern 30.

Then, as shown in FIG. 5F, the upper Si plate 201 a is subjected to afirst etching process by Deep RIE using SF6 gas and C4F8 gas. This firstetching is continued until a predetermined etching depth (say, 5 μm) isachieved in the surface of the upper Si plate 201 a. Instead of the DeepRIE, wet etching using KOH solution may be employed.

Then, as shown in FIG. 5G, the second mask pattern 50 is removed by theapplication of an organic solvent or by exposure to oxygen plasma. Theorganic solvent should be reactive on the second mask pattern 50 but(substantially) nonreactive on the first mask pattern 30. Examples ofsuch organic solvent are tripropylene glycol methyl ether, aminoethylethanolamine, phosphoric acid aqueous solution, or a mixture ofmonoethanolamine and dimethyl sulfoxide. For instance, when the firstmask pattern 30 is made of SiO2 and the second mask pattern 50 is madeof Si3N4, use may be made of phosphoric acid aqueous solution for theselective removal of the second mask pattern 50 only (i.e., the firstmask pattern 30 remains).

Then, as shown in FIG. 5H, a second etching process is performed, withonly the first mask pattern 30 present, by Deep RIE using SF6 gas andC4F8 gas. This etching process is continued until an etching depth of 95μm is achieved in the upper Si plate 201 a. If necessary, anover-etching is performed for an additional depth (e.g. 1 μm) tocompensate for a processing error.

With the above described steps, the upper Si plate 201 a is formed withcomponents or elements which correspond to the mirror forming base 210of the micromirror unit 200, the first comb-teeth electrodes 210 a-210b, the inner frame body 221, the third comb-teeth electrodes 221 a-221b, the upper frame member 231, the first torsion connectors 240, thetorsion bars 251, and the total of eight support beams 270. Since thesecond etching process is performed by Deep RIE, the torsion bars 241and 251 are rendered nonuniform in thickness so that their ends areprovided with a curved portion serving the prevention of stressconcentration.

Following the second etching step shown in FIG. 5H, a protection coatingforming step is carried out, as shown in FIG. 6A. The protection coatingor sacrificial coating 70 encloses the components formed in the upper Siplate 201 a, so that these components will not be broken during thesubsequent steps of the fabrication procedure. The protection coating 70may be formed by applying molten glass to the upper plate 201 a and thenannealing the glass material. Instead of a glass material, acommercially available resist material such as AZ or TSCR may be appliedto the upper Si plate 201 a to form a protection coating. It is alsopossible to stick a film sheet to the plate 201 a. In light of thecontrollability of adhesion timing, the film sheet may preferably bemade of a UV material which cures upon exposure to ultraviolet light.

After the protection coating 70 is formed, the lower Si plate 201 b isprocessed in the following manner.

First, though not shown in the figures, a third etching mask layer isformed on the exposed surface (lower surface in FIG. 6A) of the lower Siplate 201 b. The third etching mask layer is made of silicon dioxide andhas a thickness of 100-1000 nm. Then, the third layer is etched toprovide a third mask pattern 31. This etching is performed with the useof a third mask 41 shown in FIG. 8A. The configuration of the third mask41 corresponds to the paired electrode bases 222, the second comb-teethelectrodes 222 a-222 b, the first through the fourth auxiliary strips233-236, and the fourth comb-teeth electrodes 232 a-232 b.

Then, as shown in FIG. 6B, a fourth mask pattern 51 is formed on thelower Si plate 201 b. The fourth mask pattern 51 is made by forming afourth etching mask layer (photoresist layer) on the lower Si plate 201b, and then etching this layer into the predetermined pattern with theuse of a mask 61 shown in FIG. 8B. The thickness of the fourth etchingmask layer may be 0.5-50 μm.

Then, as shown in FIG. 6C, the lower Si plate 201 b is subjected to afirst etching process. The first etching is performed by Deep RIE withthe use of SF6 gas and C4F8 gas. The etching process is continued untila desired etching depth (say 5 μm) is attained.

Then, the fourth mask pattern 51 is removed by the application of anorganic solvent or by exposure to oxygen plasma, while the third maskpattern 31 remains intact. Thereafter, as shown in FIG. 6D, the lower Siplate 201 b is subjected to a second etching process. The second etchingis performed by Deep RIE using SF6 gas and C4F8 gas, and is continueduntil a desired etching depth (say 95 μm) is attained. If necessary, anover-etching is carried out for an additional depth (e.g. 1 μm) tocompensate for a processing error.

With the above steps, the lower Si plate 201 b is formed with componentsor elements which correspond to the electrode bases 222, the secondcomb-teeth electrodes 222 a-222 b, the lower frame member 232 of theouter frame 230, the fourth comb-teeth electrodes 232 a-232 b, and fourtorsion bars 252.

Then, as shown in FIG. 6E, the first mask pattern 30, the third maskpattern 31 and the prescribed portions of the insulating layer 260 areremoved by wet etching for example. Thereafter, though not shown in thefigures, a micromirror unit is cut out from the processed plateassembly, with the support beams 270 still attached. The removal of thesupport beams 270 may be performed thermally or mechanically. Forexample, each support beam 270 is formed with a cut at a prescribedportion by irradiating laser beams, and then is blown away. Instead, anelectric current may be caused to pass through the support beam 270, togenerate Joule heat for melting the support beam.

In the fabrication method described above, the mirror member 211 may beformed before the first step shown in FIG. 5A is initiated. The mirrormember 211 may be made in the following manner. First, a titanium layer(50 nm in thickness) is formed in a prescribed area corresponding to theresulting mirror forming base 210. Then, a gold layer (500 nm inthickness) is formed on the titanium layer. Finally, the titanium-goldlayer assembly is subjected to etching to be made into a prescribedconfiguration. The thus obtained mirror member 211 is reflective andelectrically conductive. Therefore, electrical connection to the supportplate (typically silicon wafer) can be made via the mirror member 211.Thus, if necessary, a connection wire can be bonded to the mirror member211.

In the above embodiment, a substrate material is formed with an openingon one hand, and a bridging portion remains in the substrate on theother. To achieve this, a first etching process and a second etchingprocess are performed. In the first etching process, the first and thesecond mask patters are used, so that the substrate material is etchedaway until the predetermined thickness of the bridging portion isattained. Then, a second etching process is performed with the use ofonly the first mask pattern as a mask. As a result, the substratematerial, two components are connected to each other via only thebridging portion.

According to the present invention, a torsion connector may be made invarious forms, as shown in FIGS. 9A-9I. In each figure, a plan view(left) and a sectional view (right) are shown. The sectional view istaken along two-headed arrow lines.

Specifically, referring to FIG. 9A, the torsion connector includes onlyone torsion bar 310. As illustrated, the torsion bar 310 has arelatively wide left end and a relatively narrow right end. From left toright, the torsion bar 310 becomes monotonically smaller in width. Themaximum value of the width may be in a range of 30˜300 μm, while theminimum value of the width may be in a range of 1˜30 μm. As only partlyshown in the sectional view, the torsion bar 310 is solid throughout itsentire length.

When the torsion bar 310 is used in a micromirror unit of the firstembodiment for example, the left end of the bar 310 is connected to themirror forming base 111, whereas the right end is connected to the frame113. The same connecting manner holds for the other torsion connectorsshown in FIGS. 9B.about.9I.

Referring to FIG. 9B, the torsion connector also includes only onetorsion bar 320. The torsion bar 320 is the smallest in width (1˜30 μmfor example) at its intermediate point, but is the greatest in width(30˜300 μm for example) at its right and left ends. As proceeding fromthe left end to the intermediate point, the width of the torsion bar 320becomes monotonically smaller, while it becomes monotonically greaterfrom the intermediate point to the right end.

Referring to FIG. 9C, the torsion connector 330 includes two nonparalleltorsion bars 331, 332. The connector's width (W), which is defined bythe torsion bars 331-332, becomes gradually smaller from the left end tothe right end. As seen from the accompanying sectional view, the twotorsion bars 331, 332 are offset from each other in the thicknessdirection of a micromirror unit. When used in the micromirror unit 200of the second embodiment, one torsion bar 331 may connect the innerframe 221 to the first (upper) frame member 231, while the other torsionbar 332 may connect the electrode base 222 to the second (lower) framemember 232.

Referring to FIG. 9D, the torsion connector 340 is made in a bifurcatingform resembling a letter X. As proceeding from the left end to themidpoint, the width of the connector 340 becomes gradually smaller, butfrom the midpoint to the right end, it becomes gradually greater.

Referring to FIG. 9E, the torsion connector 350 includes three torsionbars 351, 352 and 353. The central bar 352 is connected to the objectsat right angles. The outer bars 351 and 353 are nonparallel to eachother and to the central bar 352. As in the torsion connector 330 (FIG.9C), the connector 350 as a whole tapers from left to right.

Referring to FIG. 9F, the torsion connector 360 includes two torsionbars 361 and 362. Each of the torsion bars 361, 362 is provided withflaring right and left ends for preventing stress concentration at theconnecting portion of the torsion bar to the object.

Referring to FIG. 9G, the torsion connector 370 includes two torsionbars 371 and 372. As seen from the accompanying sectional view, each ofthe torsion bars 371, 372 has an empty space inside.

Referring to FIG. 9H, the torsion connector 380 includes two torsionbars 381 and 382. As seen from the accompanying sectional view, each ofthe torsion bars 381, 382 has an elliptical cross section.

Referring to FIG. 9I, the torsion connector 390 is made in a bifurcatingform resembling a letter Y. The width of the torsion connector 390becomes gradually smaller from the left end to an intermediate point(the junction of three branches). Between the intermediate point and theright end, the torsion connector 390 has a constant width.

The present invention being thus described, it is obvious that the samemay be varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the present invention, and allsuch modifications as would be obvious to those skilled in the art areintended to be included within the scope of the following claims.

1. A micromirror unit made from a laminate which includes a firstconductive layer, a second conductive layer, and an insulating layerinterposed between the first and second conductive layers, themicromirror unit comprising: a first frame including a frame bodyoriginating from the first conductive layer, an electrode baseoriginating from the second conductive layer, and an insulating portionoriginating from the insulating layer; a mirror forming base originatingfrom the same first conductive layer as the frame body of the firstframe and provided with a mirror surface; and a first torsion connectororiginating from the same first conductive layer as the mirror formingbase and the frame body of the first frame for connecting the frame bodyof the first frame to the mirror forming base, the first torsionconnector defining a first axis about which the mirror forming base isrotated relative to the first frame; wherein the first torsion connectorincludes at least two torsion bars each having a first end connected tothe mirror forming base and a second end connected to the frame body ofthe first frame, the first ends of said at least two torsion bars beingspaced from each other by a first distance, the second ends of said atleast two torsion bars being spaced from each other by a second distancewhich is smaller than the first distance; and wherein the electrode baseof the first frame is formed with comb-teeth electrodes originating fromthe second conductive layer, the mirror forming base being also formedwith comb-teeth electrodes originating from the same first conductivelayer as the mirror forming base, the first torsion connector and theframe body of the first frame for operative cooperation with thecomb-teeth electrodes of the electrode base.
 2. The micromirror unitaccording to claim 1, further comprising a second frame and a secondtorsion connector, wherein the second torsion connector connects thesecond frame to the first frame and defines a second axis about whichthe first frame and the mirror forming base are rotated relative to thesecond frame.
 3. The micromirror unit according to claim 2, wherein thesecond torsion connector includes at least two additional torsion barseach having a first end connected to the first frame and a second endconnected to the second frame, the first end of said at least twoadditional torsion bars being spaced from each other by a thirddistance, the second ends of said at least two additional torsion barsbeing spaced from each other by a fourth distance which is smaller thanthe first distance.
 4. The micromirror unit according to claim 1,further comprising a first potential conducting path and a secondpotential conducting path, wherein each of said at least two torsionbars is connected to one of the first and the second potentialconducting paths.
 5. The micromirror unit according to claim 1, whereinspacing between said at least two torsion bars becomes monotonicallysmaller from the mirror forming base to the first frame.
 6. Themicromirror unit according to claim 1, wherein each of said at least twotorsion bars has one of a rectangular cross section, a circular crosssection and an elliptical cross section.
 7. The micromirror unitaccording to claim 1, wherein each of said at least two torsion bars hasa hollow structure.
 8. The micromirror unit according to claim 1,wherein at least one of the first and second ends of each torsion barhas a curved portion for prevention of stress concentration.